Sea cucumbers change the stiffness of their skin, Venus flytraps roll up their leaves and even pine cones are capable of closing up their scales at increasing levels of humidity. In the course of evolution, Nature has managed to give rise to complex materials capable of responding to external stimuli by way of mechanical movement. Which is exactly what chemists are now trying to do as well - and with considerable success! Dr. Jiayin Yuan's team at the MPI of Colloids and Interfaces in Golm, Germany, recently scored a particularly exciting breakthrough. The researchers managed to synthesize a membrane capable of rolling up extremely rapidly when exposed to fumes.

Now, Prof. Dr. Joe Dzubiella, a theoretical physicist at the HZB, has managed to identify those factors that are responsible for the high speed.

The porous material, which looks like a sponge, consists of interconnected polymers. Here, polymers in the top layers are visibly more tightly interconnected than is true of the bottom layers. And when the membrane takes up certain gas molecules like acetone, it bulges more strongly at the top than it does at the bottom, so that it starts to warp, ultimately coiling up.

Simple diffusion and geometry

"Jiayin Yuan and his team have already characterized the phenomena in much depth, and we were able to simply pick up where they left off," explains Dzubiella, prompting him to propose that the gas molecules initially cross the membrane by simply "diffusing" across it. The time it takes them to penetrate the membrane is described by the law of diffusion and depends both on the size of the pores as well as on the thickness of the membrane. The larger the pores and the thinner the membrane, the faster the gas molecules are able to cross. The chemists had already witnessed this behavior, which the law of diffusion describes in quantitative terms, in the lab.

Dzubiella also managed to show why it is that the membrane literally curls up when exposed to the vapour, in other words, why it exhibits a particularly small radius of curvature: "We're talking simple geometry," he says. "If the membrane is really thin, very small expansions of the top layers are enough to cause strong bending." Within one-tenth of a second, the membrane bends into a full circle; within a half a second, it is rolled up multiple times. Which is ten times faster than is true of similar materials.

Membrane also reacts on perfume

Together with his postdoc, Dr. Jan Heyda, Joe Dzubiella is currently in the process of continuing his work using a computer to simulate the movement and embedding of gas molecules within the membrane's network. The reason being that, at a microscopic level, the processes are rather complex and especially between the polymeric molecules and the gases, very different types of interactions are able to take place. As such, the polymer network also takes up water molecules from the humidity in the atmosphere. If the membrane now comes into contact with acetone, the acetone molecules migrate into the network, displacing the water molecules. "Often times, it is only by way of simulations that we're able to show the ways in which this could be happening and which processes and factors are important here. These insights in turn are helping the chemists optimize a given parameter in the lab in order to reach the desired property," explains Dzubiella.

In terms of potential applications, really, the sky's the limit. For example, you might coat other types of materials with these kinds of membranes, which begin to fold up as soon as they come into contact with certain molecules. Already, the chemists were able to document that the rolling up not only works with pungent acetone but even with French perfume!

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